Radio beacon



Feb- 5 1945- c. w. EARP 2,394,157

RADIO BEACON Filed Feb. e, 194s Patented Feb. 5, 1946 RADIO BEACON Charles William Earp, London, England, assigner to Standard Telephones and Cables Limited, London, England, a British company applicati/n February c, 194s, serial No. 474,951 In Great Britain April 3, 1942 3 Claims.

The present invention relates to arrangements for determining the bearing of a mobile radio receiver with respect to a radio beacon.

A dependable omnidirectional radio beacon, giving accurate geographic bearings to a mobile receiver is an ideal which would offer a great advance in the art f aerial navigation. Systems have been proposed and used, but have not become widely adopted owing to complexity of apparatus, slowness in operation, or lack of dependability.

The system most widely used to date is the rotation of the directional eld pattern of a transmitter, by the mechanical rotation of its aerial system. A timing pulse is also radiated at those instants when the null point of the directive field pattern` is pointed in a particular direction, say north. The bearing is obtained by timing the period between the North pulse, and the instantof zero signal defined by the coincidence of the null point with the bearing of the mobile vehicle. Such systems are costly, and are slow in operation, owing to the low maximum speed of rotation obtainable. on a large aerial structure.

Purely electrical systems have also been proposed, often giving exactly the same eld pat.- tern which would be obtained by a quick rotation of an antenna structure. While providing the necessary speed of operation for use by aircraft, and for cathode ray oscillograph indication of bearing, such systems have depended for accuracy, upon an accurate balance between power radiated from diiierent aerials and delivered by diiierent valves.

It is the object of this invention to provide a radio beacon system which is purely electrical, offering high speed of operation with socalled instantaneous cathode ray indication of bearing, while maintaining the accuracy and reliability of the semi-mechanical systems.

According to the invention arrangements for determining the bearing of arnobile radio receiver with respect to a radio beacon comprise a radio beacon system in which the phase of a transmitted wave varies progressively around the transmitting antenna system and means is provided in the receiver for determining the phase of the received waves.

According to a feature of the invention means are provided for transmitting a characteristic signal at a predetermined instant of the wave period, preferably when the amplitude of the wave vpasses through zero.

In carrying out the invention according to one embodiment the beacon comprises two directional antenna systems, for example, two pairs of antennaev each arranged at the ends of the diagonals of a square, or two perpendicularly related loops fed with constant amplitude continuous Waves of frequency f1 n such a way as to make the phase ofthe radiated frequency vary with the angle o the bearing. This is achieved by feeding the two pairs of antennae or loops with equal currents phased by The radiated field is circular in pattern, but lines of constant` phase are not circles, as in the case of radiation `from a single antenna, but are connectedup into a single spiral which advances one wave-length from the aerial system for each convolution. At any fixed distance from the transmitting aerial system, the phase of the field rotates uniformly over 360 round the complete circuit defined by such distance.

If, now, we can measure the phase of the received wave as compared with the phase which would be received by a receiver situated due North of the transmitter (but at the same distance) we shall be able to deduce the geographic bear- The comparison signal for. phase measurement is provided as follows:

l. A second frequency f2, differing by a small amount from f1, by 50 cycles, for example, is radiated from an omnidirectional aerial.

2. Either f1 or f2 or both are momentarily cut off at (f1-fz) times per second to provide the necessary comparison signal or zero instant for phase measurement.

Now a receiver picks up both f1 and fz, and a beat note (f1-f2) is produced at the detector. The phase of this beat-note rotates uniformly round any circle whose centre is located at the transmitter antenna system in the same Way as the phase of f1 rotates.

The necessary comparison signal to provide a reference or zero instant for phase measurement is derived from the sharp negative pulse produced by the momentary interruption of the transmission. In order that such measurement can always indicate the bearing, the negative or timing pulses are applied in such manner that they are received at a receiver situated, say, due north of the beacon, at the same instants as the beat-note (f1-f2) passes through zero ampIitude.

field round a circle whose centre is at the beacon shall progress round that circle. In practice, simple loops would not be used. On waves of medium or long wave lengths the loops would be replaced by pairs of vertical aerials fed in phase opposition, in order to avoid the radiation of horizontal polarisation. On very short waves it may be desired to make use of horizontal polarisation, in order to avoid reflections from near obstructions. In this case, the loops could be replaced by horizontal dipoles and the omnidirectional radiation f2 could be made from a horizontal loop.

The invention will be further describ'edin conjunction with the accompanying drawing beacon transmitter, and in Figure 2 a receiver anode supply voltage, for example.

for use in conjunction Ywith the transmitter of- Figure 1.

of frequency f1 feeds into the power amplifier A1, which in turn feeds a pair of crossed loop antennae HI, H2, excitation of the two loops being fed through phase splitting or other arrangement PS. A Y

A second master oscillator O2, of frequency fz feeds the power amplifier A2 which in turn feeds the vertical omnidirectional aerial V.

The reference or timing pulse is added to the transmission as follows: Y Y

O1 and O2 both feed into detector D, where the beat frequency fi--fz is generated. This frequency (f1-fz) may be amplified and filtered in the band-pass lter F1. The pure sine-wave thus developed is subjected to an amplitude limiter `(L1)Y which produces a square wave-form which is fed to high-pass lter F2 which produces in its output a succession of sharp alternate positive and negative pulses, equally spaced in time. There are `(f1-f2) positive pulses per second and (f1-.Jz) negative pulses per second. These pulses occur at the moments when the amplitude of the wave from which they are produced passes through zero.

The positive pulses are now removed by a unidirectional amplitude limiter L2, which may be a single unit rectifier shunting the positive pulse. The train of negative pulses are now applied to overbias and interrupt the operation of the power amplifiers A1 and A2 causing interruptions in the transmissions from Hl, H2 and V. It may be arrangedY that the interruptions in the transmission occur at the desired instant by a .phase control in known manner of the beatfrequency (f1-f2) before pulse generationin L1 and F2 or by a phase control of the H. F. input either to HI and H2, or the vertical aerial V.

If it is feared that the amplifiers A1 and A2, orv

ceived signal, namely the beat-signal produced between f1 and fz.

The beat frequency fi-fz is filtered oif in the bandpass lter F3, and this frequency is applied Adirectly to one pair of plates of vra cathode ray oscillograph CR, and to the other pair of plates via a'phase shifter PS2 which produces Va phase shift of The result of these two inputs to the oscillograph is a circular bright trace.

The output from DI is also fed t'o a high-pass filter vFi which selects the pulse wave-form caused 'by the momentary'interruption of the total transmission. After suitable amplification, if necessary, thispulse train is appliedto a control electrodeV indicated at CE of the oscillograph CR. This control electrode may be a focussing grid, in which case the electron beam may be brought into correct focus only at the coincidence of pulses, when the bearing is indicated as a bright spot of light at the periphery of the oscillograph scales., Alternatively, the received pulses may beappliedto control the deflection sensitivity of the oscillograph by modulation of the In this case Vthe indication is a permanent bright circle with a sharp radial kink at the correct bearing.

Errors may be introduced by a slight drift of oscillator frequencies Yat the transmitter, vresulting in an appreciablechange in the beat frequency fi-fz. Such aLdrift will inevitably produce a variable phase shift in filter F3 in the receiver,`and this might well produce a corresponding error on the oscillog'raph screen.

Such error is entirely avoided by the use-of an identical filter F1 at the transmitter for generation of the pulse train. By this means phase distortion ofthe steady tone (f1-f2) rin the receiver is exactly compensated by a similar time distortion in the transmitted pulse train. By means of this arrangement a variable beat-note .f1-fzV may be used in order` to confuse, and prevent use by an unauthorized user.

It is desirable that the omnidirectional singlephase radiation shall be made from an antenna which, in the horizontal plane,'is placed at the centre of the diphase radiating system. The path lengths from receiver toA each aerial systeni mustbe equal in 'oi-der to avoid a "semi-circular erro-r. f

Whilst a particularr embodiment of theinvention has been described by way of example, other embodiments will occur to those skilled inthe art, such embodiments falling within the scope of the appended claims.` l f What is claimed is:

1. A direction finding system including a radio beacon transmitting station and a mobile receiving station, said transmitting station comprising a plurality of angularlyrelated antenna 'systems constituting a firstoinni-directional antenna system, means for producing high frequency electrical energy of frequency f1 of-fwhich 'portions are fed to each antenna system in relative 'phases to produce a rotating field about said omni-directional antenna system, a second omni-directional antenna system of constant field phasel around the system, means for producing high frequency electrical energy of frequency'fz fed to said Vsecond omni-directional antenna system, I neans for selectively interrupting the transmission of frequencies f1 and f2 when'tl'ie beat frequency' produced therebetween passes tlir'cuglil zero/ ampli- ,-tude, means for obtaining said beat frequency -and applying the same to'producesharply'defined, square wave, negative electrical pulses when said beat frequency passes through a predetermined Vamplitude andV means to Yapply said negative pulses to selectively interrupt the'transmission of Vfrequencies f1 and fz', said means for VproducingV sharply defined, square wave, negative pulses including a detector 4fedlwith,"frequencies f1 and f2, 'thereby producing a'b'eatfsine wave,

an amplitude limiter fed thereby and converting said beat Wave into a square Wave form, a highpass lter fed with said square Wave and producing therefrom a succession of sharp alternate duce therefrom said negative pulses, a cathode ray oscillograph at said mobile receiving station and a lter at said receiving station for passing the frequency i-z to said oscillograph, said filters being identical, whereby errors due to frequency drifts at the transmitting station are eliminated.

3. A direction finding system according to claim 1, in which are included master oscillators constituting the means for producing high frequency electrical energy of frequencies f1 and f2, and biased amplifiers fed by said oscillators, and in which said square Wave negative pulses are applied t0 the grid bias portions of said amplifiers so as abruptly to cut off the respective outputs thereof.

CHARLES WILLIAM EARP. 

